The way in which exhaust valve failure is caused by the ash in crankcase oils is not well understood. This study provides an in-depth analysis of the failure mechanism. It includes: a field test, dynamometer tests, deposit and metallurgical analysis.The valves fail due to high-temperature oxidation and erosion. This results from a loss of deposit from the valve seat. There was a direct relation between the level of sulfated ash in the oil and the time to valve failure.The paper provides recommendations on the type of oils required to prevent these failures in two-cycle engines.